1. Field of the Invention
The present invention relates to a pilger die and a pilger mandrel which are used to produce a dashpot tube having an outer diameter of 31.75 mm or less and an inner diameter 25.654 mm or less by processing a zirconium alloy tube (a material tube reduced extrusion, TREX) in two pilgering processes (a first step pass and a second step pass) using a single pilgering apparatus, and methods of manufacturing the pilger die and the pilger mandrel, and a dashpot tube for a nuclear fuel assembly.
2. Description of the Related Art
Generally, zirconium alloy tubes are widely used in the frameworks of nuclear fuel assemblies, including nuclear-fuel-rod-cladding tubes, guide thimble tubes, instrument tubes, dashpot tubes, sleeves, etc. In other words, the zirconium alloy tubes are critical elements for nuclear fuel assemblies installed in light-water reactor type or heavy-water reactor type nuclear power plants.
In such nuclear fuel assembly frameworks, the dashpot tubes have an outer diameter less than that of the guide thimble tubes. The dashpot tubes function to control the speed at which control rods fall when the control rods are inserted into the nuclear fuel assemblies, thereby absorbing the shocks applied to the control rods. Furthermore, the dashpot tubes absorb external forces generated when the nuclear fuel assemblies are inserted into nuclear reactors, or when the nuclear reactors are being operated, thus reducing the shocks applied to the nuclear fuel assemblies.
Recently, the development of an improved nuclear fuel assembly for nuclear power generation has instigated the creation of a guide thimble tube which is an element of the improved nuclear fuel assembly and is designed by a new method (a tube-in-tube method). Here, a dashpot tube is required to be manufactured in such a manner that a zirconium alloy tube, such as a material tube reduced extrusion (TREX) having an outer diameter of 63.5 mm and an inner diameter of 41.66 mm is formed into an intermediate product having an outer diameter of 31.75 mm and an inner diameter of 25.65 mm, and then formed into a final product having an outer diameter of 22.6 mm and an inner diameter of 21.23 mm, by a 50LC (a model number for pilgering machine) pilgering apparatus. During the process of manufacturing the dashpot tube, each pilgering pass includes a washing process, a pickling process and a heat treatment process.
The pilgering, which includes the multi-step pilgering passes, establishes not only measurements of the dashpot tube but also its physical properties. In detail, the zirconium alloy tube which is the dashpot tube has radius-directional textures. If the textures of the zirconium alloy tube are oriented in the radial direction of the tube, hydrogen embrittlement is induced when the tube is installed in the nuclear reactor and the reactor is operating. Therefore, the textures of the zirconium alloy tube must be oriented in the circumferential direction, that is, in which the textures extend along the circumference of the tube.
The orientation of the textures of the zirconium alloy tube is designated as a factor Q (=(ln[t0−t2]/t0)/ln(D0−D2)/D0) which is a natural logarithm ratio of a reduction of a thickness to a reduction of the outer diameter during the pilgering process. Here, t0 and D0 respectively denote the thickness and the outer diameter of the tube before the pilgering is conducted, and t2 and D2 respectively denote the thickness and the outer diameter of the tube after the pilgering has been completed. As the value of the factor Q increases, the compression efficiency is enhanced. When the factor Q is 1 or more, the textures of the zirconium alloy tube are oriented in the circumferential direction. Hence, in the conventional technique for processing the zirconium alloy tube, the factor Q is 1 or more and, typically, it is designed to fall within the range of from 1 to 4, taking into account the compression efficiency.
Furthermore, to improve the operating performance and the quality of the nuclear fuel assembly to bring it in line with the development of the nuclear fuel assembly using the above-mentioned dashpot tube, a dashpot tube having a relatively small diameter, for example, having an outer diameter of 22.6 mm and an inner diameter of 21.23 mm, which cannot be pilgered by the conventional 50LC pilgering apparatus, is required.
That is, in the conventional technique, there is neither a pilger die nor a pilger mandrel that can manufacture a dashpot tube having a relatively small diameter, for example, having an outer diameter of 22.6 mm and an inner diameter of 21.23 mm, using the 50LC pilgering apparatus. Thus, after a zirconium alloy tube is pilgered into an intermediate product having an outer diameter of 31.75 mm and an inner diameter of 25.65_mm by the 50LC pilgering apparatus, the intermediate product is transferred to a separate 25LC pilgering apparatus, because the 50LC pilgering apparatus cannot form a dashpot tube which has a diameter and thickness less than these values. Thereafter, the 25LC pilgering apparatus pilgers the intermediate product, thus producing the dashpot tube having the outer diameter of 22.6 mm and the inner diameter of 21.23 mm.
As such, to manufacture the relatively-small-diameter dashpot tube which cannot be produced by the 50LC pilgering apparatus, the intermediate product pilgered by the 50LC pilgering apparatus must be transferred to the 25LC pilgering apparatus to conduct the second pilgering process. Every time after a pilgering process is conducted, the washing process, the pickling process and the heat treatment process must be carried out. As a result, the time taken to manufacture the dashpot tube is increased, and thus, the production cost of the dashpot tube is increased.
Furthermore, in the case where a dashpot tube having a relatively small diameter, for example, having an outer diameter of 22.6 mm and an inner diameter of 21.23 mm, is manufactured by pilgering the first-pilgered zirconium alloy tube using the 25LC pilgering apparatus, it is difficult to increase the factor Q to 4 or more to ensure structural integrity of the tube. In addition, there are other problems, in that the measurements of the tube are not uniform, it is not easy to control the orientation of the textures of the tube and, when the nuclear fuel assembly provided with the dashpot tube having the above-mentioned problems is installed in the nuclear reactor, the stability of the operation of the nuclear reactor is substantially reduced.
Therefore, the development of a pilger die and a pilger mandrel is required which make it possible to manufacture the dashpot tube having a relatively small diameter, for example, having an outer diameter of 22.6 mm and an inner diameter of 21.23_ mm, only using the 50LC pilgering apparatus, without transferring an intermediate product from the 50LC pilgering apparatus to the 25LC pilgering apparatus.